This invention generally relates to a reproducing system for a video signal reproducing apparatus, and particularly to a system in which vertical vibrations of a reproduced image on a television screen are avoided during slow or still reproduction.
A well known commercial television system uses an interlaced scanning system consisting of repeating frames having a first, or odd, field which reproduces alternate lines of each frame and a second, or even, field interlaced with the first field, which reproduces the remaining lines of each frame. The odd field consists of parallel television lines, the first of which begins in the upper left corner of the television screen and the last of which ends in the middle of the bottom of the screen. The even field consists of parallel television lines interlaced between the lines of the odd field, the first of which begins in the middle of the top of the television screen and the last of which ends at the lower right corner of the television screen. The video at a point in a line from an even field, for example, contains video data which reproduces a part of the picture occurring just above or just below a corresponding point on the next adjacent line from the odd field. The vertical distance between such corresponding points is one half pitch, or one half the center to center distance between adjacent field lines (which is equal to the vertical center to center distance between adjacent interlaced lines).
When video signals of the same field are repeatedly reproduced by a video tape, video sheet or video disc reproducing device, direct use cannot be made of the reproduced synchronizing signal since this signal lacks the necessary relationship between vertical and horizontal synchronizing pulses for producing interlaced scanning. A locally generated synchronizing signal having the necessary relationships between horizontal and vertical synchronizing signals is normally produced in the reproducing apparatus and substituted during reproduction for the corresponding recorded signals. However, the use of locally generated synchronizing signals may result in the video signal being displayed in one field on the television screen and then the same video signal being again displayed but shifted up or down by an amount corresponding to one half a pitch. Accordingly, the reproduced picture appears to vibrate annoyingly up and down on the screen even though the interlaced raster is stationary.
The shifting process described above occurs when processing a composite color television signal, but an additional problem arises from the fact that the phase of a chrominance component in the composite color television signal in the NTSC system has a specific and changing relationship from line to line, field to field, and frame to frame. Specifically, the phase of the color subcarrier is reversed from line to line and from field to field. The locally generated synchronizing signal may contain a color burst signal having the correct changing phase relationship to frames and fields to correctly demodulate a color signal. When repetitively scanning the same field line, however, the phase of the reproduced chrominance signal may not be in the relationship required by the NTSC system, and a correct color image cannot be reproduced. The above requirement as to the changing relationship of phase is especially important when generating a composite color television signal for broadcasting.